Recently, image forming apparatuses such as printers have been given color capability and used as various expressing means by users. In particular, color page printers are attracting attention because they are silent and capable of high-quality, high-speed printing.
A multicolor beam printer as one color page printer is characterized by printing a multicolor image by performing first development by scanning a light beam on a photosensitive body in a main scan direction, and then transferring the image onto a printing medium such as a printing paper sheet on a transfer carrier to perform predetermined processing.
A method of printing a multicolor image by this multicolor beam printer will be described below with reference to FIGS. 18 and 19.
FIG. 18 is a schematic view of a conventional multicolor beam printer. FIG. 19 is a block diagram of signal processing.
Referring to FIG. 18, a photosensitive drum 201 which rotates in the direction of an arrow at a predetermined constant velocity is charged to a predetermined polarity and a predetermined voltage by a charger 204.
Printing sheets P are fed one by one at a predetermined timing from a paper feed cassette 215 by a paper feed roller 214. When a sensor 202 senses the leading edge of the printing sheet, a laser beam L modulated by an image signal VDO is emitted from a semiconductor laser 205 toward a polygonal mirror 207.
This laser beam L is scanned by the polygonal mirror 207 and guided onto the photosensitive drum 201 via a lens 208 and a mirror 209.
A signal (to be referred to as TOPSNS hereinafter) from the sensor 202 placed at one end of light scan is output as a vertical sync signal to an image processor 250 (FIG. 19).
The image signal VDO is sequentially supplied to the semiconductor laser 205 in synchronism with a BD signal (to be described later) which follows the TOPSNS signal. When the laser beam L enters a detector 217, a beam detection signal (to be referred to as a BD signal hereinafter) serving as a horizontal sync signal is output.
The polygonal mirror 207 is driven by a scanner motor 206. This scanner motor 206 is controlled by a motor control circuit 225 so as to rotate at a predetermined constant velocity in accordance with a signal S2 from a frequency divider 221 which divides the frequency of a signal S1 from a reference oscillator 220 shown in FIG. 19.
The photosensitive drum 201 is exposed by scan in synchronism with the BD signal, and a developing device 203Y develops a first electrostatic latent image. After that, a first toner image of yellow is formed on the photosensitive drum 201.
Immediately before the leading edge of the printing sheet P fed at a predetermined timing reaches a transfer start position, a predetermined transfer bias voltage having a polarity opposite to that of toner is applied to a transfer drum 216. Consequently, the first toner image is transferred onto the printing sheet P, and at the same time this printing sheet P is electrostatically attracted to the surface of the transfer drum 216.
Subsequently, a second electrostatic latent image is formed on the photosensitive drum 201 by manipulating the laser beam L. A developing device 203M develops this second electrostatic latent image to form a second toner image of magenta on the photosensitive drum 201. This second toner image is transferred onto the printing sheet P so as to be aligned with the position of the first toner image previously transferred onto the printing sheet P. Note that the end of the image of each color is defined by the TOPSNS signal.
Analogously, a third electrostatic latent image is formed and developed by a developing device 203C, and a cyan toner image formed is aligned with and transferred onto the printing sheet P. A fourth electrostatic latent image is then formed and developed by a developing device 203K, and a black toner image formed is aligned with and transferred onto the printing sheet P.
As described above, a VDO signal of one page is output to the semiconductor laser 205 in each step. Also, whenever the transfer step is performed, a cleaner 210 scrapes off any untransferred toner image.
After that, when the leading edge of the printing sheet P on which the toner images of four colors are transferred approaches the position of a separation pawl 212, this separation pawl 212 comes in contact with the surface of the transfer drum 216 to separate the printing sheet P from the transfer drum 216. The end portion of this separation pawl 212 keeps contacting the transfer drum 216 until the trailing edge of the printing sheet P is separated from the transfer drum 216. After that, the separation pawl 212 moves away and returns to the original position. A charger 211 removes stored charge on the printing sheet P to facilitate separation of the printing sheet P by the separation pawl 212, and reduces air discharge during separation.
FIG. 20 is a timing chart showing the relationship between the TOPSNS signal and the VDO signal described above. Referring to FIG. 20, reference symbol A1 denotes a printing operation of the first color; A2, a printing operation of the second color; A3, a printing operation of the third color; and A4, a printing operation of the fourth color. These sections A1 to A4 form a color printing operation of one page.
FIG. 21 is a block diagram showing the system configuration of a conventional printer.
Referring to FIG. 21, a printer 302 receives a control signal and an image signal 307 from an external apparatus, e.g., a host computer 301. A printer controller 303 transfers the control signal to a printer control unit 304. The image signal is supplied to a laser driver 310 of a printer engine via an image processor 305 in the printer controller 303 and drives a semiconductor laser 306.
FIG. 22 is a block diagram showing the internal arrangement of the image processor 305 shown in FIG. 21. The image processor shown in FIG. 22 receives an image signal of 8 bits for each of R, G, and B, i.e., a total of 24 bits from the printer controller (not shown). A color processor 351 converts each of Y, M, C, and K signals into the 8-bit VDO signal described above at respective timings (FIG. 23 is a corresponding timing chart).
A γ correction unit 325 converts these Y, M, C, and K VDO signals into γ-corrected, 8-bit signals and inputs these signals to a pulse width modulation unit 353 (to be referred to as a PWM unit hereinafter) in the next stage. In this PWM unit 353, a latch 345 synchronizes the 8-bit image signal with the leading edge of an image clock iVClK. A D/A converter 355 converts the signal into an analog voltage and inputs the voltage to an analog comparator 356.
The image clock iVCLK is converted into a triangular wave by a triangular wave generator 358 and input to the analog comparator 356. This analog comparator 356 compares the two signals and outputs an image signal 309 subjected to PWM. An inverter 357 inverts the output signal to obtain a desired PWM signal.
FIG. 24 shows a timing chart when the PWM unit 353 generates a PWM signal. As shown in FIG. 24, when the input 8-bit image data to the PWM unit 353 is FF[H] (H indicates hexadecimal notation), the widest PWM signal is output. When the image data is 00[H], the narrowest PWM signal is output.
Unfortunately, the improved printing performance and high-quality printing capability of a conventional image forming apparatus as described above lead to frequent occurrence of forgery of securities such as paper money.
As the image formation technology improves in the future, the image quality improves accordingly, so this sort of crimes are expected to increase in number.